Chronological skin aging is a complex process that is controlled by numerous intrinsic and extrinsic factors. One major factor is the gradual degradation of the dermal collagen fiber network. As a step toward understanding the mechanistic importance of dermal tissue in the process of aging, this study employs analytical and multiscale computational models to elucidate the effect of collagen fiber bundle disintegration on the mechanical properties and topography of skin. Here, human skin is modeled as a soft composite with an anisotropic dermal layer. The anisotropy of the tissue is governed by collagen fiber bundles with varying densities, average fiber alignments, and normalized alignment distributions. In all finite element models examined, collagen fiber bundle degradation results in progressive decreases in dermal and full-thickness composite stiffness. This reduction is more profound when collagen bundles align with the compression axis. Aged skin models with low collagen fiber bundle densities under compression exhibit notably smaller critical wrinkling strains and larger critical wavelengths than younger skin models, in agreement with in vivo wrinkling behavior with age. The propensity for skin wrinkling can be directly attributable to the degradation of collagen fiber bundles, a relationship that has previously been assumed but unsubstantiated. While linear-elastic analytical models fail to capture the postbuckling behavior in skin, nonlinear finite element models can predict the complex bifurcations of the compressed skin with different densities of collagen bundles.

中文翻译：

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胶原降解对皮肤力学行为和皱纹的影响

按时间顺序排列的皮肤老化是一个复杂的过程，受众多内在和外在因素的控制。一个主要因素是真皮胶原纤维网络的逐渐降解。作为了解皮肤组织在衰老过程中的机械重要性的一步，本研究采用分析和多尺度计算模型来阐明胶原纤维束解体对皮肤机械性能和形貌的影响。在这里，人体皮肤被建模为具有各向异性真皮层的柔软复合材料。组织的各向异性由具有不同密度、平均纤维排列和归一化排列分布的胶原纤维束控制。在所有检查的有限元模型中，胶原纤维束降解导致真皮和全层复合材料刚度逐渐降低。当胶原束与压缩轴对齐时，这种减少更为明显。与年轻皮肤模型相比，在压缩下具有低胶原纤维束密度的老年皮肤模型表现出明显更小的临界皱纹应变和更大的临界波长，这与随年龄增长的体内起皱行为。皮肤起皱的倾向可直接归因于胶原纤维束的降解，这种关系以前曾被假设但未经证实。虽然线性弹性分析模型无法捕捉皮肤的后屈曲行为，但非线性有限元模型可以预测具有不同密度胶原束的压缩皮肤的复杂分叉。